A plain old block cipher, key and cleartext-block in, ciphertext-block
out, is said to be in Electronic Code Book (ECB) mode. A given block
of plaintext always encrypts to the same block of ciphertext. This
makes it somewhat vulnerable to known plaintext attacks, block replay
attacks, etc.

A fairly cheap alternative is to use it in Cipher Block Chaining (CBC)
mode. All this does is XOR each plaintext block with the previous
ciphertext block before encryption. For the first block, where there
is no previous ciphertext block, a caller-specified Initialization
Vector (IV) is used for the XOR. This makes each block's encryption
depend on all the previous blocks